1. Field of the Invention
The present invention relates to an image forming apparatus which forms multiple dots on a recording medium by ejecting ink droplets including a pigment as a colorant, and an image forming method using the apparatus.
2. Description of the Related Art
Recently, according to a rapid progress of an ink jet technology, it is possible to perform a large size color printing which enables both a high speed and a high image quality using an image forming apparatus of an ink-jet method. Particularly, the apparatus is used in various fields in sign and display applications, and it is also possible to be applied to printing of an over-the-counter point of purchase (POP) advertisement or a poster, outdoor advertisement, a billboard or the like. It is possible to obtain printed materials in the ink-jet method, by forming multiple dots by ejecting multiple types of ink droplets on a recording medium. That is, it is possible to realize many colors on the recording medium by variously combining the dots with different size or color.
In the meantime, in a case where the ink including a pigment is used, since a particle diameter thereof is relatively large compared with dyes, inclusions such as the pigment are likely to remain on a recording surface. Particularly, there are problems that non-uniformity of gloss occurs in a gradation image, and as a result, an appearance (grade) of the image is degraded. This is because a concavo-convex shape occurs even in the same image due to discreteness and discontinuity of a dot arrangement rule. Thus, various image forming methods are proposed according to which the occurrence degree of gloss is approximately even in each density gradation range.
JP2010-284951A proposes controlling a surface smoothness of the image by changing a position where the ink droplets are applied. In FIG. 17 of JP2010-284951A, there is described that variation of the gloss over the whole density gradation range is decreased, by decreasing the surface smoothness (gloss) of the low density gradation range.
In JP2008-162095A, there is described that a mask pattern with high (or low) dot dispersiveness for each path is used, with respect to an ink combination with higher (or lower) gloss during image recording.